1. Field of the Invention
This invention is directed to sensor devices that utilize carbon nanotubes as a chemically sensitive element and methods for using carbon nanotubes as a chemically sensitive element. The invention is also directed to methods of making devices with carbon nanotubes as a chemically sensitive element.
2. Description of the Background
Techniques and devices for detecting a wide variety of analytes in fluids such as vapors, gases and liquids are known. As used herein the term “fluid” means gases, vapors and liquids. An electronic nose is an instrument used to detect vapors or chemical analytes in gases, solutions, and solids. In certain instances, the electronic nose is used to simulate a mammalian olfactory system. In general, an electronic nose is a system having an array of sensors that are used in conjunction with pattern-recognition algorithms. Using the combination of chemical sensors, which produce a fingerprint of the vapor or gas, the recognition algorithms can identify and/or quantify the analytes of interest. The electronic nose is thus capable of recognizing unknown chemical analytes, odors, and vapors.
In practice, an electronic nose is presented with a substance such as an odor or vapor, and the sensor converts the input of the substance into a response, such as an electrical response. The response is then compared to known responses that have been stored previously. By comparing the unique chemical signature of an unknown substance to “signatures” of known substances, the unknown analyte can be determined. A variety of sensors can be used in electronic noses that respond to various classes of gases and odors.
A typical type of sensor is disclosed in U.S. Pat. No. 5,571,401, issued to Lewis et. al; the disclosure of this application is hereby incorporated by reference. Devices utilizing the sensor disclosed by Lewis et. al. are described in U.S. Pat. Nos. 6,450,008; 6,422,061; 6,418,783; 6,085,576; 6,234,006; and 6,319,724 and U.S. Published patent application Ser. Nos. 09/796,877; 10/099,405; 10/174,633; and 10/153,883, the disclosure of these patents and applications are hereby incorporated by reference.
The sensors disclosed by Lewis et al. are individual thin-film carbon-black polymer composite chemiresistors configured into an array. The collective output of the array can be used to identify an unknown analyte using standard data analysis techniques. Each individual detector of the sensor array is a composite material consisting of conductive carbon black homogeneously blended throughout a non-conducting polymer.
The detector materials are deposited as thin films on an alumina substrate each across two electrical leads creating conducting chemiresistors. The output from the device is an array of resistance values as measured between each of the two electrical leads for each of the detectors in the array.
When a composite is exposed to a vapor-phase analyte, the polymer matrix acts like a sponge and “swells up” while absorbing the analyte. The increase in volume is concomitant with an increase in resistance because the conductive carbon-black pathways through the material are broken. When the analyte is removed, the polymer “sponge” off-gasses and “dries out”. This causes the film to shrink and the conductive pathways are reestablished. The baseline resistance (Rbaseline) of the device is measured while a representative background vapor flows over the array. The response from the chemiresistor during an analyte exposure is measured as a bulk relative resistance change (ΔRmax/Rbaseline). Since an analyte will absorb into the different polymer matrices to different degrees, a pattern of response is observed across the array.
The relationship between volume increase and the resistance change in these composite films can be described by percolation theory. See Lonergan, M. C.; Severin, E. J.; Doleman, B. J.; Beaber, S. A.; Grubbs, R. H.; Lewis, N. S. Chem. Mater. 1996, 8, 2298. Percolating networks are ubiquitous in nature and are based on the idea that certain phenomena propagate through a system by interactions between neighboring active sites. Examples include forest fires spreading from tree to tree, disease spreading through a population by human contact, sol-gel transitions in biopolymer gelation, and, as in this case, resistor networks that are either continuous, and connected, or discontinuous. (See Stauffer, D.; Aharony, A.; Introduction to Percolation Theory; Taylor & Francis: Bristol, Pa., 1994).
In percolating resistor networks, there is a critical point where the last conductive pathway is broken and the system becomes discontinuous. This point is called the percolation threshold and is highly dependent on system variables. In the case of carbon black polymer composites, the percolation threshold is reached when an individual detector's resistance sharply increases with a small increase in volume of the composite film. The form of a percolation curve is very similar to a pH titration curve. The change in volume required to reach the percolation threshold for a given detector is dependent on the amount of carbon black in the polymer matrix, the structure of that carbon black, the degree of contact between carbon black clusters, and how homogeneously dispersed are the carbon black clusters through the matrix.
The polymer matrix “swells up” because analyte vapor absorbs into the film to an extent determined by the partition coefficient of the analyte. The partition coefficient defines the equilibrium distribution of an analyte between the vapor phase and the condensed phase at a specified temperature. This is expressed as: K=Cs/Cv (1); where Cv is the concentration of the analyte in the vapor phase, and Cs is the concentration of the analyte in the condensed phase, which is also proportional to the detector's response. Therefore, the larger an analyte's partition coefficient, the more it will absorb into a polymer film, and the larger will be the detector's response.
Each individual detector element requires a minimum sorbed amount of analyte (Cs,min) to cause a response noticeable above the baseline noise. However, the minimum vapor concentration (Cv,min) needed to produce Cs,min is different for each analyte since the partition coefficient is different for each analyte. Moreover, it can be shown with standard thermodynamic arguments, that the magnitude of response of an individual detector can be predicted to first order by the fractional vapor pressure exposed to the detector irrespective of the analyte identity. See Atkins, P. W.; Physical Chemistry; W. H. Freeman and Co.: New York, N.Y., 1994; and Doleman, B. J.; Severin, E. J.; Lewis, N. S. Proc. Natl. Acad. Sci., USA 1998, 95, 5442. Therefore, the general detection limit of a sorption device is best expressed as a minimum fraction of equilibrium vapor pressure rather than a concentration value.
The behavior of the detector to be sensitive to the fractional vapor pressure of the analyte it is exposed to explains why sorption devices are generally rather insensitive, in terms of concentration, to high vapor pressure analytes like methane (which is a gas at ambient temperatures) and diethyl ether, but show good sensitivity, in terms of concentration, to low vapor pressure compounds exposed at low concentrations such as volatile fatty acids. For example, if the limit of detection were 0.1% of an analyte's vapor pressure, this would indicate a detection limit of 74 ppm for ethanol, but only 0.5 ppm for nonanal (a common taint in packaging materials) at 24° C. All analytes will have roughly the same limit of detection when expressed as a fractional vapor pressure.
The differences between detector responses when exposed to a given analyte—which are required to uniquely identify that analyte by providing a unique response pattern—are due to differences in chemical interactions between the analyte and the detector films. Therefore, the limit of discrimination between two analytes exposed at the same fractional vapor pressure is determined by their relative collective chemical differences across the array. This indicates the need for as much chemical diversity as possible in the polymers comprising the array detectors for a general-purpose electronic nose. Moreover, for well-defined applications, the polymers used in the detector array can be chosen to maximize chemical differences between target analytes to increase the discrimination power of a smaller array.
As previously discussed, although the sensors disclosed by Lewis et al. work relatively well for low vapor pressure/large molecule analytes the sensors are relatively insensitive to high vapor pressure/small molecule analytes. Accordingly, a need exists for a sensor that is more sensitive to small molecule analytes.
The conductivity of carbon nanotubes is known to change with exposure to oxygen. For example, as part of a broad program investigating the synthesis, characterization and theoretical modeling of carbon nanotubes, scientists with the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley have reported that the electronic properties of these tubes are so “extremely sensitive” to oxygen that exposure to air can convert a semiconducting nanotube into a metallic conductor.
“Many supposedly intrinsic properties measured on nanotubes may be severely compromised by extrinsic air exposure effects,” the scientists state. See Collins et al., “Extreme Oxygen Sensitivity of Electronic Properties of Carbon Nanotubes”, Science 2000 287: 1801-1804.
One of the co-authors of the Science article stated that “We've demonstrated that carbon nanotubes can behave as both n-type and p-type semiconductors. Until now, all nanotube measurements had suggested p-type conducting behavior only.” In the Science article, the Berkeley researchers describe that the degree of oxygen exposure is the determining factor as to whether a carbon nanotube functions as an n-type or p-type semiconductor.
Another one of the Science article's co-authors suggests that in principle, a nanotube's electronic properties could be controlled through the use of “protective coatings” to shield select portions of the nanotube from oxygen exposure.”
Prevailing theories have held that the electronic properties of a nanotube are dictated solely by the diameter and chirality (geometric configuration) of the tube. Theories also predicted that natural defects in the hexagonal web of a nanotube's carbon atoms (nanotubes are essentially tiny sheets of graphite that have been curled and connected along a seam like a drinking straw) would give rise to the creation of atomic-sized electronic devices, a prediction that experiments in 1997 by Zettl and Collins, both authors of the Science article, confirmed.
In their latest study, Zettl and his associates found that the chemical environment surrounding a nanotube is at least as important an influence on the tube's electronic properties as its diameter. Working with single-walled carbon nanotubes (SWNTs) grown by conventional laser ablation methods, the researchers studied both bulk samples and single isolated tubes. Measurements of both electrical resistance and thermoelectric power, the voltage induced by a temperature gradient, were made under environmental conditions that gradually shifted from oxygen to vacuum and back to oxygen.
“The effects of oxygen exposure became increasingly more irreversible (and have longer time constants) with decreasing temperature, as expected for a gas adsorption process,” the scientists state in the Science article. “In fact, our transport measurements indicate that, once SWNTs have been exposed to oxygen, it is not possible to fully deoxygenate them at room temperature even under high vacuum conditions.”
Further evidence that the effects being observed were the result of gas adsorption came when the topology of the nanotubes was changed. Dilute SWNT thin films yielded quick electronic changes, while optically thick films required higher temperatures and longer times to reach equilibrium.
The experiments were repeated with different major gas constituents of air to confirm that the changes in electronic properties were due to oxygen adsorption. Carbon materials such as charcoal are known for their excellent adsorption and sieving properties, but nanotubes were thought to have been an exception because of their morphology, especially the smoothness of their exterior surface. However, a sensor utilizing carbon nanotubes to sense the presence of various chemical species has not yet been accomplished.